Every time rents jump, a vacancy rate gets blamed. It's the figure journalists reach for, the one buyers' agents quote, the one that supposedly tells you whether a suburb is tight or loose. But almost nobody who repeats it can tell you how it's actually built — and that's a problem, because the way it's measured changes what it means.
The one-line definition
A rental vacancy rate is the share of rental properties sitting empty and available to lease at a given moment, measured as a percentage of all the rentals in a market.
That's it. The arithmetic isn't where the confusion lives.
What the number is really telling you
Think of the vacancy rate as a pressure gauge, not a property count.
When the rate is low, there are few homes available for the people who want them. Tenants compete, queues form at inspections, and rents climb. When the rate is high, the boot is on the other foot — landlords compete for tenants, and rents soften.
So what's low and what's high? The industry reads the gauge in three rough zones: under about 2% is a tight market that favours landlords and pushes rents up; 2% to 3% is considered roughly balanced; and above 3% starts to tilt towards tenants. Worth being clear about one thing — these bands are rules of thumb the trade has settled on, not thresholds handed down by any official body. Treat them as a guide, not gospel.
A low number means few rentals are available relative to demand, so tenants compete and rents climb. A high number flips it — landlords compete for tenants, and rents ease.
Why a suburb full of empty homes can still show zero vacancy
Walk through some inner-city pockets at night and you'll see them: whole floors of dark windows, apartments that never seem to be lived in. So how can the same suburb post a "tight" vacancy rate? Because the gauge isn't measuring emptiness. It's measuring how much of the rental market is genuinely up for grabs.
Here's the distinction that trips people up. The vacancy rate does not count every empty dwelling — only rentals that are actually being offered to tenants and sitting unleased. A standard rental that's been advertised and hasn't found a tenant counts. A holiday house that's never listed doesn't. A home idle between owners doesn't. And the big one most people are picturing — empty owner-occupied homes and investment properties deliberately left vacant and never put up for rent — doesn't count either.
This is why a suburb full of dark windows can still post a low vacancy rate — most of those homes were never in the rental pool to begin with.
That's why empty and "vacant" aren't the same word here. A street can be full of unlit homes and still feed almost nothing into the rental pool the vacancy rate actually measures.
Where it gets messy: defining "vacant"
To turn that tidy percentage into a real number, someone has to pin down two slippery things: what counts as vacant, and how big the total rental pool actually is. Neither is sitting in a government register waiting to be read off. Both have to be estimated.
Take SQM Research, one of the most widely cited providers. It builds its count from online rental listings, watching the major listing sites across a full calendar month. Ads with no real address or unique ID get thrown out. The same property listed on two sites is counted once, not twice.
Then comes the rule that does almost all the work:
Only those listings that have been advertised for three weeks or more are used.
It sounds like a technicality. It isn't. Plenty of rentals are listed and snapped up within days, and if you counted those, you'd be logging homes as "vacant" that were never really empty. The three-week filter strips them out. Change that one rule — count every fleeting listing instead — and the same market would suddenly look far more vacant than it is. A single judgement call, quietly steering the headline number.
The three-week filter is the one that matters most: it strips out rentals snapped up in days, which would otherwise inflate the vacancy rate with homes that were never really empty.
The other half of the sum: how big is the rental pool?
The bottom of the fraction is just as slippery as the top. To work out what share of rentals are vacant, you need to know how many rentals exist in the first place — and, oddly, no one keeps a live count. There's no national register ticking over in real time, logging which homes are rented out and which aren't.
So SQM falls back on the most reliable headcount we have: the ABS Census, which records how many households are renting in each postcode. The catch is that the Census only runs every five years — 2021 is the latest — so the figure is already years old the moment it's used. To cover the gap, the months in between are estimated by filling in between the five-yearly counts.
Which means the denominator isn't a hard tally either. It's a careful estimate, built on solid foundations but stretched to fit the present.
Why two "official" vacancy rates can disagree
Here's what catches people out: there isn't one vacancy rate. There are several, and they don't always agree. For the same city in the same month, one might read 1.7% while another sits closer to 2%.
Neither is wrong. They're counting in different ways.
Most providers watch the ads. SQM Research, Domain and Cotality (the data firm formerly known as CoreLogic) all build their figures from online rental listings — broadly, the rentals advertised and still unleased, measured against an estimate of the total rental pool. But even these three differ in the detail: SQM, for instance, only counts a listing once it's been advertised for three weeks or more, a filter the others don't apply the same way.
The REIA takes a different road entirely. Its numbers come from the state real estate institutes — bodies like REINSW — which survey their member agencies on how much of the property they manage is sitting empty. That's a rent-roll headcount, not a tally of online ads.
So when two "official" rates clash, it usually isn't a mistake. One counted listings, the other surveyed agents. Which is why the takeaway isn't to hunt for the "true" figure — it's to pick one provider and follow their series over time. A rate that's been climbing for six months tells you something real. The same number held up against a rival's, measured a different way, mostly tells you they used different methods.
Three providers watch the ads; one surveys agents. Feed in different data and set different rules, and the same market produces different numbers — which is why it pays to follow one provider's trend rather than compare across them.
The bottom line
The vacancy rate is a genuinely useful signal — arguably the cleanest read we have on rental supply and demand. But it's an estimate wearing the costume of a hard fact. Know that, and the number becomes a tool rather than a headline.
Sources
SQM Research, Domain, Cotality, REINSW, and the Australian Bureau of Statistics.
